Frequency stabilizing system



May 3, 1960 Q. A. KERNs E'I'Al-` FREQUENCY STABILIZING SYSTEM Filed July24, 1957 INVENTORIS. QUENTIN A. KERNS By OSCAR A. lANDERSON ATTORNEY.

United States Patent "P FREQUENCY STABILIZING SYSTEM Quentin A. Kems,Orinda, and Oscar A. Anderson, Oakland, Calif., assignors to the UnitedStates of America as represented by the United States Atomic Energy4Commission Application July 24, 1957, Serial No. 673,989

5 Claims. (Cl. 328-133) The present invention relates to a frequencysynchronizing system and, more particularly, to an electronic circuitresponsive to the frequency difference between two concurrentfrequencies.

(Ihe present invention is useful for comparing two frequencies,producing an output synchronizing signal for frequency correction and/or an indication of the magnitude fand sign of frequency difference. Theinvention is readily adaptable for synchronizing shaft rotation speedsinelectrical machinery, either with a reference standard or with anothermachine. llhrough the usage of higher frequency type components, theinvention may also be utilized for frequency synchronization of radiofrequency generators.

1The invention utilizes two signal channels, the first channel providinginformation indicative of the extent of the difference between the twoinput frequencies while the second channel provides informationindicating which of the two input frequencies is higher. The systemproduces an output signal having a magnitude proportional to thedifference between the two frequencies while the polarity of the signalindicates whether the compared frequency is higher or lower than thereference frequency. By having a minimum of capacity in the circuit, therespouse time is made quite low. d

vVIhe system is sensitive only to frequency differences, thereby makingthe output independent ofV relative phase angle between the frequenciesbeing compared. The absolute phase angle may be controlled byconventional means, the present invention being adapted for control ofrate of phase change.

Clt is an object of the present invention to accurately compare twofrequencies, providing an output signal suitable for corrective controlor for indication.

' lIt is another object of the invention to provide a means forsynchronizing a variable frequency generator with a reference frequency.

It is a further object to provide a means for holding two or moreindependently variable frequency generators in synchronism with eachother while Varying in absolute frequency, or maintaining a constant ora programmed difference frequency.

'The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification` taken inconjunction with the accompanying drawing, which is a circuit diagram ofthe system with typical waveforms being indicated.

The operation of the overall system will be first described by referenceto the component circuits in block form as indicated by dashed lines inthe drawing, followed by a more detailed account of individual circuits.

Referring now to the drawing, there is shown a variable generator 11which produces an output frequency which is. to be controlled. Areference generator 12 has an output frequency to which the output ofthe variable generator'11 is to be compared. It shouldY beunderstoodthat the reference generator 12 need notfhave a constant out- 2,935,686Patented May 3, 1960 put frequency, but may be another variablefrequency first adder 16, producing first adder output waveforms 1'7 and17. 'The first adder waveform 17, shown here, results when the generatorwaveform 13 and reference waveform 14 are at different frequencies sothat the first adder waveform 17 consists of alternate cancellations andadditions at a rate equal to the frequency difference. The first adderwaveforms 17 and 17' are identical except for a 180 phase differential.In all instances in the present discussion where a prime number is usedto label a waveform, it may be assumed to be identical to thecorresponding waveform except for a 180 phase difference.

The first adder waveforms 17 and 17' are separately rectified'in a firstrectifier 18, producing rectied wave-` forms19 and 19 waveshapesfollowing the envelope of the voltage peaks in first adder waveforms 17and 17. The rectifier 18 includes filtering means for removing thehigher frequency components from the rectifier output waveforms 19 and19. The rectified waveforms 19 and 19 are added together, resulting in atriangular waveform 20 which is differentiated in a differentiator 21 toproduce square waveshape differentiator waveforms 22 and 22'. Thedifferentiator waveform 22 has an essentially steady state negativevalue when the triangular waveform 20 has a negative or downward slopeand has a positive value when the triangular waveform 20 has a positiveor upward slope. The amplitude of the differentiator waveform 22 isproportional to the steepness or angle of the slope of the rectifiedwaveform 20. The output of the differentiator 21 is then applied to agated rectier 23 which will be described in greater detail hereinafter.

The output of the variable generator 11 is shifted 90 in a phase shifter24, which may be formed by the conventional resistance-capacitance phaseShifters at the higher frequencies. When the invention is utilized tomaintain synchronization of the shaft speed of rotating machines, theshaft position may be conveniently indicated by utilizing a spur toothrotor aixed to the shaft for inducing a signal in a proximal stationarymagnetic pickup which becomes the variable generator 11. -A secondstationary magnetic pickup may be used to conveniently obtain a secondsignal displaced out of phase with the first signal. In the followingdiscussion it should be understood that such a system might be utilizedinstead of the phase shifter 24.

The output from the phase shifter 24 is combined with the output of thereference source 12 in a second adder 26, identical in operation to thefirst adder 1-6. However, since the output of the Variable generator 11has been shifted 90, assumed here to be a lagging phase shift, theenvelope of the second adder output waveform 27 will be shifted 90 withrespect to the envelope of the first adder waveform'17. The second adderwaveforms 27 and 27 are shown here 90 leading the first adder waveforms17 and 17', respectively. Reference to a particular lagging or leadingphase shift is, of course, only for purposes of example and it isintended that nothing inherent in the invention should be impliedthereby.

A second rectifier 28, identical to the first rectifier 18, is coupledto the second adder 26 and produces second rectifier output curves 29and 29. The outputs of the second rectifier 28 are coupled to asummarizer circuit I:31 wherein the secondk rectifier curves 29 and 29'are signals 32 and 32. The outputs of the summarizer circuit 31 arecoupled to the gated rectifier 23 which also receives the outputs of thedifferentiator 21. The gatedl rectifier 23 produces a direct currentgated rectifier output signal 33, available at an output terminal 34.The gated rectifier V23 in this example produces a positive voltageoutput when the two input signals (diiferentiator. curve 22 andsummarizer curve 32) are in phase, but will produce a negative outputsignal when the two input signals are of opposite phase. The amplitudeof the output signal will be dependent on the amplitude of thedifferentiator curve 22, which in turn has an amplitude proportional tothe frequency differential between the variable generator 11 andreference 12.

The output signal 33 at the output terminal 34 may be coupled to thevariable generator 11 as a frequency adjusting feedback signal,indicated by a dashed line 36. However, the output signal may beutilized in other ways or combination of ways as previously discussed.

There is shown in detail the circuitry of the differentiator 21 andgated rectifier 23 for a particular embodiment of the invention. Therectifier waveform 19 is applied to the input dierentiator 21, thesignal passing through a high resistance differentiator resistor 49 inseries with one end of a low impedance center-tapped primary winding 51of a transformer 52. Similarly, the rectifier waveform 19' is passedthrough a second diferentiator resistor 53 to the opposite end of theprimary winding S1. The differentiator resistors 49 and 53 form theprincipal portion of an essentially constant impedance in thedifferentiator input circuit, thus the current through the primarywinding S1 produces a magnetic field proportional to the currentdifferential of the rectifier waveforms 19 and 19'. Rectifier waveforms19 and 19 are combined in the transformer 52 so fthat'the primary 51current essentially follows the triangular waveform 20. A center-tappedsecondary winding S4 in the transformer 52 has equal but oppositepolarity differentiator output curves 22 and 22. In this instance thecurves 22 and 22 are indicated with respect to the secondary center-tappotential rather than to a steady state ground potential.

In the gated rectifier 23 there are eight rectifier tubes connected inpairs, whereby the cathodes of a first, third, fifth, and seventhrectifiers 56, 57, 58, vand 59 are connected to the anodes of a second,fourth, sixth, and eighth rectifier 61, 62, 63, and 64, respectively. Arst connecting wire 66 couples the differentiator output curve 22 fromthe differentiator 21 to the cathode of the first rectifier S6.Similarly, a second connecting Wire 67 couples the differentiator outputcurve 22 to the cathode of the fifth rectifier 58. A center-tap lead 68of the transformer 52 is connected to the output terminal 34 and througha capacitor 69 to ground. The cathodes of third and seventh rectifiersS7 and 59 are connected to ground. A first rectifier lead 71 couplessummarizer curve 32 through a decoupling resistor 72 to the anodes ofthe fifth and seventh rectifiers 58 and 59, and similarly through adecoupling resistor 73 to the cathodes of the second and fourthrectiiers 61 and 62. A second rectifier lead 74 couples the summarizercurve 32' through a decoupling resistor 76 to the cathodes of the sixthand eighth rectifiers 63 and 64, and similarly through a decouplingresistor 77 to the anodes of the'rst and third rectifiers S6 and 57.

Considering now the operation of the gated rectifier circuit, it will beseen that during the first half cycle represented by the summarizercurves 32 and 32', lthe first, second, third, and fourth rectifiers 56,57, 61, and 63 will be conductive while the remaining four rectifiers58, 59, 63, and 64 are non-conductive. Therefore, in effect the secondconnecting wire 67 from the differentator 21 is connected to an opencircuit. However, thel first connecting wire 66 is effectively connectedto ground 4 Y through the low impedance of the conducting second andfourth rectifiers 61 and 62. Since at this moment the potential at thecenter-tap lead 68 will be positive (the inverse of differentiator curve22), the charge across the capacitor 69 and the output signal 33 at theoutput terminal 34 will be positive during the first half cycle ofoperation. The amplitude of the output signal 33 is dependent on theamplitude of the differentiator curve 22. Considering the second halfcycle of operation from 180 to 360, the summarizer output waveforms 32and 32 will cause the fifth, sixth, seventh, and eighth rectifiers 58,63, 59, and 64 to be conducting while the remaining rectifiers 56, 61,57, and 62 are non-conducting. Therefore, the rst connecting wire 66leads to an open circuit while the second connecting wire 67 isconnected t0 ground through the low impedance of conducting fifth andseventh diodes 58 and 59. Thus the potential at the center-tap lead 68will again be positive (the inverse of the last 180 of differentiatorcurve 22') and the steady state positive output signal 33 is obtained.

' If, in the previous example, the variable generator waveform 13 isassumed to have a frequency higher than the reference generator waveform14, then in the alternate case where the frequency of the variablegenerator waveform 13 is lower than the reference generator waveform 14,the summarizer waveforms 32 and 32 will be shifted with respect to thediiferentiator curves 22 and 22'. By reapplying the events in theprevious example, it will be apparent that the output signal 33 willthen be negative.

Consider now the circuitry of the first adder 16 and the first rectifier18. The variable generator 11 is cou' pled to the primary winding 81 ofa variable generator transformer 82 having two secondary windings 83 and84 with equal number of turns and identical electrical characteristics.Similarly, the reference generator 12 is coupled to the primary winding86 of an identical reerence transformer 87 having two secondary windings88 and 89. Secondary winding 83 of the variable generator transformer 82and secondary winding 88 of the reference transformer 87 are seriesconnected. The remaining secondary windings 84 and 89 of the twotransformersv are similarly connectedY in series, but in the oppositesense or phase relationship. This means that at a particular moment, twoinput signals to the variable generator and reference generatortransformers 82 and 87 are being added in phase in secondary windings 83and 88, but are being added in the inverse phase in secondary windings84 and 89. Therefore, the envelope of the adder output waveform 17 fromthe secondary windings 83 and 88 is 180 out of phase with the envelopeof the adder output waveform 17 from the secondary windings 84 and 89.

In the rectifier 18, the first adder waveform 17 is rectified by a firstdiode 91 and high frequency components are removed by a first filtercapacitor 92, producing rectifier waveform 19, which is the envelope ofthe first added waveform 17. Similarly, the first adder waveform 17' isrectified by a second diode 93 and high frequency components removed bya second filter capacitor 94, producing rectifier output waveform 19.Thus, the rectifier waveforms 19 and 19' are related by a. 180 phasedifferential.

The summarizer circuit 31 contains a summarizer transformer 96 having ahigh impedance center-tapped primary winding 97 wherein the secondrectifier Waveforms 29 and 29' are combined to produce a pair oftriangular summarizer output waveforms 32 and 32.' across acenter-tapped secondary winding 98. A load resistor 99 is connectedacross the secondary Winding 98. The effect of the loaded secondarywinding 98 and the high Vimpedance primary winding isto maintain anapproximately constant load to the gated rectifier 23 when thev currentpath changes from one bank of rectier tubes-to the other.

Considering now thel method by which the'invention detects Whether thevariable generator ll'isroperating at a frequency higher or lower thanthe reference generator 12, an explanation willbe presented by usingexamples. First, assume two equal amplitude signals having frequenciesf1 and f2, comparable to variable generator signal 13 and referencesignal 14where f1 is higher than f2. At a time T, f1 will be inphasewith f2, producing a resultant signal having maximum amplitude equal tothe sum of the two signals. At a time T+l80 the signals f1 and f2 cancelto give zero output. At a still Alater time T+360 and signals f1 and f2again are in phase, A`adding together as at time T. 'Ihe resultant wouldbe comparable to the first adder waveform 17. InV the second adder,assume f1 has been shifted ahead 90 in the phase shifter 24, therefore,the inphase time when ;f1+90 and f2 combine to form a maximum amplituderesultant signal must have occurred 90 before the time T. Thus at time Tthe resultant signal is decreasing toward zero. By time T+180 the zerooutput condition will have been reached and the output signal will beincreasing toward a maximum, thereby forming a signal with thecharacteristic of second adder waveform 27.

Assume now a second example where f1 is a lower frequency than f2. Asbefore, at time T the signals f1 and f2 will add inphase, while later attime T-l-l80 signals f1 and f2 will cancel. At time T|360 the signalswill again add, thus the first adder waveform 117 is produced exactly asin the rst example. However, in the phase shifter Z4, it is f1, thelower of the two frequencies that has been shifted ahead 90. Shiftingthe lower frequency f1 ahead 90 is equivalent to shifting the higherfrequency f2 back 90. Ihus in the first example the higher frequency f1was shifted ahead of the lower frequency f2 and in the second examplethe higher frequency f2 was effectively shifted back of the lowerfrequency f1. Since the higher frequency f2 is in effect delayed withrespect to f1, at time T, f2 (--90) will be approaching the condition ofbeing in phase with f1, producing a second adder 'output waveforminversely related to the second adder output waveform 27 of the firstexample. It follows then, that the summarizer output waveform will beinversely related to the waveform produced in the first example.Therefore, by review of the operation of the gated rectifier 23, it willbe seen that the polarity of the potential at the output terminal 34will reverse from the first example to the second example.

The embodiment of the invention described here was intended for usage atcomparatively `low frequencies. It will be apparent to those skilled inthe art that usage of the invention at other frequencies and conditionswill engender numerous variations and modifications within the spiritand scope of the invention and thus it is not intended to limit theinvention except as defined in the following claims.

What is claimed is:

1. In a circuit sensitive to a co-existing signal frequency and areference frequency, the combination comprising `first means mixing saidfrequencies to produce a first difference frequency occurring at afrequency equal to the difference between said signal frequency and saidreference frequency, means shaping said difference frequency to have anessentially triangular waveshape, a differentiator receiving saidtriangular waveshape difference frequency, rst means shifting the phaseof said signal frequency, second means mixing said reference frequencyand said phase shifted signal frequency to produce a phase-shifteddifference frequency having a period equal to said first differencefrequency, means controlled by said phase-shifted difference frequencyrectifying the output of said differentiator whereby an output potentialis produced having an amplitude proportional'to said differencefrequency and having a polarity frequency.v

' 2L In a circuitfor detecting the frequency difference between' ltwoinput signalsthe combination comprising a first mixer circuit combiningthe said two input signals and having means for providing a generallytriangularly shaped output signal with a frequency equal to thedifference between said two input signals, a diiferentiator havingan-input coupled to the output of said mixer circuit and providing agenerally square-wave signal representing the differential of the slopeof said triangularly shaped signal, phase shifting means coupled to oneof said input signals, a second mixer combining the other of said inputsignals with said phase-shifted input signal and providing a secondmixer signal having a frequency equal to the frequency differencetherebetween, a phase-sensitive circuit receiving said second mixersignal and said squarewave signal and producing an output signal havingpolarity dependent on the phase relationship therebetween and having anamplitude dependent on the amplitude of said square wave.

3. In a circuit for detecting the difference between a variablefrequency and a reference frequency, the combination comprising avariable generator transformer having a first and a second secondarywinding and having a primary winding receiving said variable frequency,a reference transformer having a third and a fourth secondary windingand having a primary winding receiving said reference frequency, saidfirst secondary winding being series connected in an aiding phase tosaid third secondary winding, said second secondary winding being seriesconnected in an opposing phase to said fourth secondary winding, a firstrectifier coupled to said first secondary winding and said thirdsecondary winding, a second rectifier coupled to the output of saidsecond secondary winding and said fourth secondary winding, mixing meanscombining the outputs of said first rectifier Vand said second rectifierand being of the type which produces a triangularly shaped outputsignal, a diiferentiator coupled to the output of said mixing meanswhich differentiator is of the class producing a square waveshape outputsignal, a phase shifter receiving said variable frequency, anadder'circuit combining the output of said phase shifter with saidreference frequency, a third rectifier receiving the output of saidadder circuit, a gated rectifier receiving the outputs of said thirdrectifier and said differentiator.

4. In a circuit for detecting the frequency difference between a firstand second alternating current input signal, the combination comprisinga first adder circuit receiving said first and second input signal andproducing a first and second beat frequency signal having beat frequencycomponents mutually related by a phase difference, a first rectifierreceiving said rst beat frequency signal, a second rectifier receivingsaid second beat frequency signal, a mixing means combining the outputsof said first and second rectifiers to produce a triangularly shapedwaveform signal, a differentator receiving said triangularly shapedwaveform signal and producing a square wave output signal, a phaseshifter receiving one of said input signals, a second adder circuitreceiving the other of said input signals and the output of said phaseshifter, and a balanced rectifier utilizing the output of said secondadder as a switching signal for rectifying the output of saiddifferentiator.

5. In a circuit for instantaneously detecting the difference between avariable frequency and a reference frequency, the combination comprisingan adder circuit combining said variable frequency and said referencefrequency to provide a first amplitude varying signal modulated at thebeat frequency between said variable frequency and said referencefrequency, said adder circuit further inversely combining said variablefrequency modulation of said'second amplitude varying signal Abeingrelated by 180 phase difference with said iirst amplitude;

varying signal, a first rectifier receiving said rst amplitude varyingsignal, a second rectifier receiving said second amplitude varyingsignal, a mixing circuit combining the outputs of said rst and secondrectifier circuits to produce an output signal having a triangularlyshaped waveform, a dierentiator receiving the output of said mixingcircuit and producing a square wave output signal, and gated rectifiermeans coupled to the output of said diterentiator.

UNrrED STATES PATENTS AHansell Jamil, Wirller Dec. 8, Smith Nov. 2,Jacobsen June 28, Crane July 17, Hawley Aug. 7, Ingalls May 6, WillardDec. 7, Patton Dec. 18,

Patton Apr. 1,

